This invention relates to on-line data processing systems and more particularly to the control for reading a magnetic tape for a recorded journal in which a hysteresis of process is recorded, at the time of a system resuming its processing after the system has failed and was restored to the normal state.
In an on-line data processing system, it is a general practice that the historical information (journal) of the processes executed therein is recorded by a magnetic tape unit.
The journal thus obtained is used chiefly for the system resuming process after the system has failed and was restored to the normal state.
The on-line data processing system performs such processes as reading a file (collected information for an object stored in a magnetic disk unit, magnetic tape unit or the like), updating the file and so on. If a failure occurs in the hardware or software of the system, the process may be stopped. At this time, the failure is removed by proper processing and the system is again started to operate. Before execution of a process, a process (system resuming process) is necessary to make the contents of the file appropriate by using the journal. In addition, to read the journal, it is absolutely necessary to fully check the magnetic tape loaded on the magnetic tape unit for a desired journal recorded therein, because the operator may have loaded an improper magnetic tape on the unit.
In general, checking of what information is recorded on the magnetic tape has so far been performed by reading out information, calling label, recorded at the beginning of the tape.
Furthermore, to restore the file to a proper state, it is necessary to first read the most recent journal.
Therefore, after the magnetic tape is rewound to its beginning where the label is read, and the presence of a proper journal is confirmed, the tape must be wound forward until the most recent journal recorded portion is read out. For this process, a time of the order of minutes is required. This means that the start of process execution is delayed by that time.